Imaging techniques play a key role in biomedical studies and clinical practice. Compared with other imaging modalities, optical methods possess several significant merits. Optical methods utilize non-ionizing and safe radiation to investigate tissue, and thus are especially suitable for screening and monitoring applications. Optical tools are also capable of providing various imaging contrasts, and therefore are versatile in visualizing different structures, physiological functions, and molecule-specific events. For example, blood strongly absorbs light; therefore, morphology of blood vasculature can be readily mapped by optical systems that exploit absorptions contrast. The difference in the absorption spectra of different hemoglobin molecules can be further exploited to evaluate an oxygen saturation level of blood. Also, blood flow can be quantified using optical frequency shifts of scattered light based on the Doppler effect. Further, when commercialized, optical systems can potentially be made compact, portable and inexpensive.
Most current optical imaging techniques can be classified into two groups. The first group, known as ballistic imaging, works in the ballistic regime. It includes early-photon imaging, confocal microscopy, and optical coherence tomography. These modalities rely on unscattered or singly backscattered photons, which are selectively collected using gating techniques based on time-of-flight, spatial collimation or coherence. Although exclusive use of ballistic photons assures high-resolution imaging, ballistic photons attenuate exponentially with penetration. As a result, imaging depth of ballistic imaging is limited to less than approximately 1.0 millimeters (mm) in highly scattering tissue, such as skin.
The second group works in a diffusive regime, and mainly includes diffuse optical tomography. Diffuse optical tomography measures diffused light reemitted from tissue through multiple source-detector pairs. An algorithm, based on a photon propagation model, is adopted to invert measurements to form a spatial map of tissue's optical properties. However, although use of diffused light allows diffuse optical tomography to visualize several centimeters deep inside turbid tissue, achieved spatial resolution is poor, typically about a fraction of a centimeter, as a result of the nature of photon diffusion.
As described above, development of optical imaging faces a major challenge, namely that turbid media, like biological tissue, strongly scatters light. Unlike X-ray photons, optical photons can penetrate approximately 1.0 mm (typical transport mean free path for biological tissue) into biological tissue, and still mostly maintain their original directions. This penetration range is called the ballistic regime. After traveling approximately 1.0 centimeters (cm) inside tissue, photons almost completely lose their memory of their original incidence direction after a large number of scattering events, and enter the so-called diffusive regime. The quasidiffusive regime (between approximately 1.0 mm and approximately 1.0 cm inside tissue) refers to the transition region between the two, where photons experience multiple scattering events and retain only a weak memory of their original directions.